Here's another that may come off the back burner someday: the Norwegian TH¡NK.
It is presently an all-electric concept, but what intrigues me is the possibility that it may be hybrid-ized by adding a supplimental "Sterling-cycle," external combustion engine to add range. A Sterling engine is much more efficient than an internal combustion engine, is virtually emissions-free, and is extremely quiet. The bubbas at Segway are working the Sterling aspect.
Sterling engines are used in some "air independant propulsion" (AIP) submarines, as they can be used while submerged. The benefit there is long endurance with no more noise than battery power.